Electroplating system for improving plating distribution of elnisil coatings

ABSTRACT

An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semiepitrochoidally aligned grooves respectively machined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

United States Patent Cordone et al.

[ 1 ELECTROPLATING SYSTEM FOR IMPROVING PLATING DISTRIBUTION OF ELNISlL COATINGS [75] Inventors: Leonard G. Cordone, Allen Park;

William A. Donakowski; John R. Morgan, both of Dearbom Heights; Karl Roemming, Detroit, all of Mich.

[73] Assignee: Ford Motor Company, Dearborn,

Mich.

[22] Filed: Nov. 5, 1973 [2]] Appl. No.: 413,154

[52] U.S. Cl. 204/272; 204/26; 204/284;

204/DIG. 7

[5 l] Int. Cl. CZSB 9/00; C25D 7/04 [58] Field of Search 204/16, 26, 38 R, 38 B,

204/DIG. 7, 272, 285, 284

[56] References Cited UNITED STATES PATENTS 3,514,389 5/l970 Stephan et al 204/237 3,640,799 2/!972 Stephan et al. 204/16 June 24, 1975 Primary Examiner-T. M. Tufariello Attorney, Agent, or FirmJoseph W. Malleck; Keith L. Zerschling [57] ABSTRACT An electroplating system for cathodically plating an epitrochoidally shaped internal surface of a rotary engine housing. An anode assembly is provided which is comprised of a perforate walled container of titanium metal or other anodically inert metal to which a voltage potential can be applied; the basket contains anode pieces such as nickel which are shaped to be in intimate contact with each other during the plating operation. The perforate walls of the anode container is shaped from flexible expanded titanium sheet metal interfitted within semiepitrochoidally aligned grooves respectively machined into titanium plates forming the ends of the anode assembly. The anode walls are thus shaped substantially complimentary to the epitrochoid configuration of the cathode but have a predetermined deviation adjacent the nodes of the trochoid for insuring a uniform but heavy coating thickness under high speed electroplating conditions.

15 Claims, 4 Drawing Figures PATENTED JUN 24 I975 SHEET 2 I3 8 91, 5 34 2 0 we 71;: Karla/yr: Z4

ELECTROPLATING SYSTEM FOR IMPROVING PLATING DISTRIBUTION OF ELNISIL COATINGS BACKGROUND OF THE INVENTION sity at any outstanding contour, while the opposite ef- 1 feet will take place at depressions. In the latter case, the density of current flow becomes less than the average density of plating current over the full area being treated. This problem becomes exaggerated when an article to be plated has a compound curvature, such as in an epitrochoid, where the cathode is able to receive current throw" from two different zones or anode locations due to the reverse or compound curvature. Accordingly, certain areas will be unduly thick because of the throwing power which is multiplied in some areas.

In applications such as a functional coating for a wear surface of an internal combustion engine, i.e. the internal rotor housing surface of a rotary internal combustion engine, the need for uniformity in the coating is extremely severe. The efficient electroplater not only seeks to obtain uniform thickness in such applications, but the plating must be of good sound density throughout; the latter will be degraded as a result of inappropriate bath chemistry, electrode spacing, and change of the anode or cathode area during the plating process.

SUMMARY OF THE INVENTION A primary object of this invention is to provide an anode assembly useful in an electroplating system for cathodically plating an article having a compound or reverse curvature, the anode assembly being particularly adapted to maintain a proper current throw relationship so that a uniform thickness and density is maintained throughout the plated surface of said article.

Another object of this invention is to provide a semiconforming anode assembly or apparatus for use in an electroplating system of the type which is adapted to deposit a significantly heavy functional coating on a non-uniformly curved surface.

Features pursuant to the above objects comprise the use of an anode assembly having a foraminous or perforate sheet metal titanium wall shaped in a predetermined unique configuration and varied from the shape of the cathode at selected locations. The wall is retained by end plates formed of the same material, but solid. The cross section of the foraminous wall is defined so that it is semi-conforming with respect to the shape of the cathode; the anode assembly progressively becomes more spaced from any portion of the cathode article which has a reversely curved portion, the pro gression of spacing increasing to a location intersected by radius of the reversely curved portion passing through the midpoint thereof.

Still another object of this invention is to provide a novel and unique method for fabricating an anode assembly which will have a defined cross sectional configuration with a semi-conforming relationship to the cathode, a continuous wall of the assembly being fabricated of perforate sheet metal, such as titanium.

BRIEF SUMMARY OF THE DRAWINGS FIG. 1 is a schematic illustration of an electroplating apparatus having a stacked series of cathodically constituted articles for plating, and an anode assembly disposed within the interior of said series of cathode articles;

FIG. 2 is a plan view of the apparatus of FIG. 1, shown similarly in a somewhat schematic manner;

FIG. 3 is a highly enlarged schematic layout of the cross sectional configuration of the anode assembly and the inner wall of the cathodic article to be plated;

FIG. 4 is an exploded view of the basic elements which interflt to form the cathode assembly according to the method of this invention.

DETAILED DESCRIPTION Turning now to the drawings and particularly FIGS. 1 and 2, there is schematically illustrated a preferred mode for an anode assembly and plating system according to this invention. An electroplating tank A is provided to contain an electrolyte B, such as an aqueous solution of nickel sulfamate containing inert particles of silicon carbide. Typically the bath may contain about 600 grams/liter of nickel sulfamate, about grams/liter of silicon carbide with a mesh size no greater than 400, about 2.5 grams/liter of a stress reliever such as saccharin, about 19 grams/liter of nickel chloride, and about 45 grams/liter of boric acid (H3303).

A cathode assembly C is disposed in the electrolyte consisting of several cathodic articles 10 each constituting a cast aluminum rotor housing useful as an element of a rotary internal combustion engine. The rotor housings are annular and must have a highly wearresistant epitrochoid surface 11 on the interior thereof and against which apex seals or other moving parts of a rotary engine must bear. The housings are separated, one from the other, by spacers 12 which may act as shields and prevent plating on the side faces 10a of the rotor housings. Such spacers can be formed as polypro' pylene or nylon sheets and have an inner edge 13 which is recessed from the interior surface 11 of each rotor housing. Alternatively, the spacers may be arranged as cathode elements which fit tightly between the housings and which draw current around the edges of the housings to overcome the problem of exaggerated thickness at such edges; again the spacers would be recessed as illustrated.

At the upper and lower ends of the stack of housings and spacers, there'is employed a rigid annular shield 14 for the top and bottom faces 15 and 16 respectively. Each shield should be a plate comprised of aluminum coated with silicone rubber which stays clean and does not draw plating. Plates are supported by a harness (not shown) which facilitates the lowering and the raising of the entire cathode assembly from the electrolyte. The harness should similarly be coated so as to have an inert outer surface.

An anode assembly D is employed which is of a semiconforming type wherein only a portion of the anode is adapted to be proportioned identical to the cathode surface 11 to be plated; other portions are designed to progressively deviate from such configuration. The anode assembly, here, is a basket made from expanded titanium sheet metal (or may be woven from titanium wire). The walls 19 are foraminous and the bottom and top walls and 21 are each a solid titanium plate. Resilient or elastic neoprene bands 22 may be mounted about the anode wall 19 to mask off or block the current throw in certain predetermined elevation zones along the anode. assembly, particularly those areas where the edges of the cathode article would promote an uneven distribution. The masking also blocks off current throw to the spacing between the housings. Such masking is unnecessary if cathodic spacers are utilized as mentioned earlier. Active anode pieces 23, such as nickel, are collected and stacked in the basket for intimate interengagement with each other and with the basket.

An anodic film is formed on the titanium basket which affords corrosion resistance and electrical insulation, the basket thereby being rendered anodically inert. The titanium acquires a thin-dense inert oxide film which is chemically resistant to acidic electrolytes and has a high electrical resistance. The cui'rent density, of course, is controlled by the configuration of the titanium basket even though the nickel anode pieces therein are the active anode metal. Current will pass between the basket and pieces at a contact point between the anodic film and the nickel pieces; this is so even though the film on the titanium is an electrical insulation. 1

To realize the objects of this invention, the anode walls 19 are defined with a predetermined variation from the nodes 24 of the epitrochoid configuration of cathode wall 11. Such nodes or segments have a reverse curvature relative to the uniform curvature of the remaining portions 25 and 28 of the epitrochoid; in cross section, the portions 25 may substantially be arcs of circles. The cross sectional configuration of wall 19 has a pair of uniform arcuate segments 26 and 27 which are directly proportional and aligned with the segments 25 and 28 respectively of wall 11. At stations on wall 19, substantially adjacent the extremeties of the reverse curvature segments 24, varying arcuate segments 30 and 31, each of which may have a different radius from that of the uniform segments, are employed. Each of these varying arcuate segments substantially continue orextend the curvature from each of the uniformly curved segments until they meet at a juncture 32 which is on the minor axis of the epitrochoid, or in other terms, has a radius 33 of the reverse curvature segment 24 passing therethrough and through a midpoint 34 of the reverse curvature. In this manner the total segment 30 or 31 is each comprised of two arcs meeting at an abrupt juncture and thereby rendering the combination as varyingin curvature.

As shown in FIG. 3, the uniform segments 27 and 26 are formed from cricles which overlap. it is possible that for some types of epitrochoids or compound cathode surfaces, the circles should be made tangent. In any event, the deviation (distance 36 distance should be progressively varied according to the relationship whereby the deviation is inversely proportional t .D., where CD. is current density, provided such factors as the conductivity of the solution and temperature are constant.

Method of Making Anode Assembly A preferred method mode according to this invention A with respect to FIG. 4, comprises: i j

1. Prepare solid titanium end plates 44 and 45 wit identical continuous groove 46 and 47 respectively, each groove defining a semi-conforming configuration to that of the cathode, surface. In this case, the semiconforming configuration comprises two uniform arcs 40 and 41 connected by varying segments 42 and 43. The varying segments are adapted to render a predetermined deviation away from the cathode surface at these areas to promote uniform plating thickness.

2. Assemble a flexible web of titanium expanded sheet metal (having a mesh size no greater than with the longitudinal edges 50 and 51 of the web in the grooves 46 and 47 respectively. The web is overlapped upon itself at a seam 52 to define a sleeve-like wall with a uniform cross section reflecting the uniformity and deviations of said grooves.

3. Locate the ends of posts 53 and 54 in mating seats 55 in the end plates to effect a strong stable joint between said plates.

4. Stitch the seam 52 with titanium wire, and fill the assembly with a collection of nickel anode pieces.

5. Provide suitable electrical means for applying a potential to the web.

We claim as our invention:

1. In an electroplating system for cathodically electroplating an article wall having a uniform curvature interrupted by a reverse curvature at least at one location, a semi-conforming anode assembly, comprising:

a. an anodically inert enclosure having a conductive foraminous wall predominantly conforming proportionately to the wall of said article excepting said reverse curvature when disposed adjacent thereto, said foraminous wall having at least one portion shaped predominantly toconform with said unifonn curvature of said article wall and at least one other portion which extends adjacent the reverse curvature and extends beyond the start of said reverse curvature of said article, said other portion progressively deviating away from conformity with said reverse curvature and having a maximum deviation at a station aligned with an extension of a radius of said reverse curvature passing through the mid-point of said reverse curvature, and

b. a plurality of anodically active metal pieces in said enclosure in interengagement with each other and with said enclosure.

2. An assembly as in claim 1, in which said article wall and foraminous wall are each continuous.

3. An assembly as in claim 2, in which said article wall has a uniform cross section shaped as an epitrochoid.

4. An assembly as in claim 3, in which the uniform arcuate segments of said foraminous wall are part of overlapping circles viewed along a cross section of said wall.

5. An assembly as in claim 3, in which the uniform arcuate segments of said foraminous wall are part of tangent circles viewed along a cross section of said wall.

6. In an electroplating system for cathodically plating a trochoidally shaped internal surface of an engine housing, the surface being generated about an eccentric axis and having opposed nodes separating lobes defined by said trochoid configuration, the system comprising:

a. an anode assembly having a perforate sheet of anodi cally inertmetal, a solid wall of anodically inert metal forming a bottom thereof, and solid anode pieces, said perforate sheet being shaped to lie parallel to said eccentric axis, the edges of said sheet metal and any cross section of said sheet metal shape taken perpendicular to said axis defining identical configurations, each such configurations being characterized by a pair of uniformly arcuate segments being disposed with one each adjacent and interior of a lobe of said surface, the spacing between said uniformly arcuate segments and said lobe being constant throughout, and a pair of varying arcuate segments disposed one each adjacent a node of said surface and effective to interconnect one extremity each of said uniform arcuate segments with the extremity each of one of said varyin g arcuate segments, and said varying arcuate segments being spaced from said node of said surface so that the extremities of each of said varying segments is spaced at the same distance as said uniformly varying segments while the intermediate portion of said varying segments progressively become increasingly spaced from said node,

b. means providing a plating bath receiving said anode assembly and having a solution effective to chemically react with said anode pieces for constituting an electrolyte to carry out said electroplatc. means for applying a voltage between the epitrochoid surface and said perforate metal container to produce a current flow between said pieces and surface, the resultant current flow being uniformly dense throughout the spacing between said surface and arcuate segments.

7. An electroplating system as in claim 6, in which said anode assembly has said uniformly varying segments defined as segments of a circle, the centers of said circles being arranged so that their peripheries overlap, said varying segments being defined as combination of a portion from each of said circles on either side of the juncture where said circles overlap.

8. An electroplating system as in claim 6, in which the uniformly arcuate segments are comprised of portions of circles which are tangent at a common point, said varying segments being formed from other portions of said circle adjacent the point of tangency and extending to either side of said tangency point.

9. The electroplating system as in claim 6, in which said perforate metal and metal bottom is comprised of titanium, and said anodic pieces are each comprised of nickel.

10. The electroplating system as in claim 6, in which said solid wall and perforate sheet define a basket providing a constant anode area throughout the entire duration of electroplating, and the configuration of said basket being defined so as to assure constant current flow throughout the spacing between said basket and cathode article.

11. An electroplating system as in claim 6, in which said perforate sheet metal is constituted to form an elongated basket effective to extend through the interior of several engine housings, each housing being stacked together one upon another with electrically inert spacers interposed therebetween, the inner edges of said spacers being recessed with respect to the inte rior wall of each of said cathodic articles.

12. An electroplating system as in claim 11, in which said spacers are conductive to act as cathodes whereby current throw and plating about the edges of said housings is reduced due to the presence of the spacers which attract plating.

13. An electrolytic plating system as in claim 11, in which non-conductive elastic bands are mounted about said perforate wall of said anode assembly at predetermined locations to block current throw bath at the spacing between housings as well as about the edge of each surface on each housing.

14. An electroplating system having an anode, cathode article and a plating solution, the improvement comprising:

a. said cathode having an internal surface to be plated which is defined with an epitrochoid configuration, said anode having a complimentary configuration disposed inwardly thereof, one uniformly curved portion of said anode configuration being spaced uniformly from the lobes of said epitrochoid configuration on said cathode, said anode configuration having other portions deviating from said epitrochoid shape at locations substantially adjacent to the nodes of said epitrochoid, said deviation being in a radially inward direction with re spect to said anode.

15. A system as in claim 14, in which said inward deviation progresses in accordance with the relationship whereb the deviation is inversely proportional to urrent density wherein deviation represents the perpendicular distance between said cathode and anode at any one point. 

1. IN AN ELECTROPLATING SYSTEM FOR CATHODICALLY ELECTROPLATING AN ARTICLE WALL HAVING A UNIFORM CURVATURE INTERRUPTED BY A REVERSE CURVATURE AT EAST ONE LOCATION, A SEMI-CONFORMING ANODE ASSEMBLY, COMPRISING: A. AN ANODICALLY INERT ENCLOSURE HAVING A CONDUCTIVE FORAMINOUS WALL PREDOMINANTLY CONFORMING PROPPORTIONATELY TO THE WALL OF SAID ARTICLE EXCEPTING SAID REVERSE CURVATURE WHEN DISPOSED ADJACENT THERETO, SAID FORAMINOUS WALL HAVING AT LEAST ONE PORTION SHAPED PREDOMINANTLY TO CONFORM WITH SAID UNIFORM CURVATURE OF SAID ARTICLE WALL AND AT LEAST ONE OTHER PORTION WHICH EXTENDS ADJACENT THE REVERSE CURVATURE AND EXTENDS BEYOND THE START OF SAID REVERSE CURVATURE OF SAID ARTICLE, SAID OTHER PORTION PROGRESSIVELY DEVIAATING AWAY FROM CONFROMITY WITH SAID REVERSE CURVATURE AND HAVING A MAXIMUM DEVIATION AT A STATION ALIGNED WITH AN EXTENSION OF A RADIUS OF SAID REVERSE CURVATURE PASSING THROUGH THE MID-POINT OF SAID REVERSE CURVATURE, AND B. A PLURALITY OF ANODICALLY ACTIVE METAL PIECES IN SAID ENCLOSURE IN INTERENGAGEMENT WITH EACH OTHER AND WITH SAID ENCLOSURE.
 2. An assembly as in claim 1, in which said article wall and foraminous wall are each continuous.
 3. An assembly as in claim 2, in which said article wall has a uniform cross section shaped as an epitrochoid.
 4. An assembly as in claim 3, in which the uniform arcuate segments of said foraminous wall are part of overlapping circles viewed along a cross section of said wall.
 5. An assembly as in claim 3, in which the uniform arcuate segments of said foraminous wall are part of tangent circles viewed along a cross section of said wall.
 6. In an electroplating system for cathodically plating a trochoidally shaped internal surface of an engine housing, the surface being generated about an eccentric axis and having opposed nodes separating lobes defined by said trochoid configuration, the system comprising: a. an anode assembly having a perforate sheet of anodically inert metal, a solid wall of anodically inert metal forming a bottom thereof, and solid anode pieces, said perforate sheet being shaped to lie parallel to said eccentric axis, the edges of said sheet metal and any cross section of said sheet metal shape taken perpendicular to said axis defining identical configurations, each such configurations being characterized by a pair of uniformly arcuate segments being disposed with one each adjacent and interior of a lobe of said surface, the spacing between said uniformly arcuate segments and said lobe being constant throughout, and a pair of varying arcuate segments disposed one each adjacent a node of said surface and effective to interconnect one extremity each of said uniform arcuate segments with the extremity each of one of said varying arcuate segments, and said varying arcuate segments being spaced from said node of said surface so that the extremities of each of said varying segments is spaced at the same distance as said uniformly varying segments while the intermediate portion of said varying segments progressively become increasingly spaced from said node, b. means providing a plating bath receiving said anode assembly and having a solution effective to chemically react with said anode pieces for constituting an electrolyte to carry out said electroplating, c. means for applying a voltage between the epitrochoid surface and said perforate metal containEr to produce a current flow between said pieces and surface, the resultant current flow being uniformly dense throughout the spacing between said surface and arcuate segments.
 7. An electroplating system as in claim 6, in which said anode assembly has said uniformly varying segments defined as segments of a circle, the centers of said circles being arranged so that their peripheries overlap, said varying segments being defined as combination of a portion from each of said circles on either side of the juncture where said circles overlap.
 8. An electroplating system as in claim 6, in which the uniformly arcuate segments are comprised of portions of circles which are tangent at a common point, said varying segments being formed from other portions of said circle adjacent the point of tangency and extending to either side of said tangency point.
 9. The electroplating system as in claim 6, in which said perforate metal and metal bottom is comprised of titanium, and said anodic pieces are each comprised of nickel.
 10. The electroplating system as in claim 6, in which said solid wall and perforate sheet define a basket providing a constant anode area throughout the entire duration of electroplating, and the configuration of said basket being defined so as to assure constant current flow throughout the spacing between said basket and cathode article.
 11. An electroplating system as in claim 6, in which said perforate sheet metal is constituted to form an elongated basket effective to extend through the interior of several engine housings, each housing being stacked together one upon another with electrically inert spacers interposed therebetween, the inner edges of said spacers being recessed with respect to the interior wall of each of said cathodic articles.
 12. An electroplating system as in claim 11, in which said spacers are conductive to act as cathodes whereby current throw and plating about the edges of said housings is reduced due to the presence of the spacers which attract plating.
 13. An electrolytic plating system as in claim 11, in which non-conductive elastic bands are mounted about said perforate wall of said anode assembly at predetermined locations to block current throw bath at the spacing between housings as well as about the edge of each surface on each housing.
 14. An electroplating system having an anode, cathode article and a plating solution, the improvement comprising: a. said cathode having an internal surface to be plated which is defined with an epitrochoid configuration, said anode having a complimentary configuration disposed inwardly thereof, one uniformly curved portion of said anode configuration being spaced uniformly from the lobes of said epitrochoid configuration on said cathode, said anode configuration having other portions deviating from said epitrochoid shape at locations substantially adjacent to the nodes of said epitrochoid, said deviation being in a radially inward direction with respect to said anode.
 15. A system as in claim 14, in which said inward deviation progresses in accordance with the relationship whereby the deviation is inversely proportional to -> current density wherein deviation represents the perpendicular distance between said cathode and anode at any one point. 